Page 58 - Advances in bioenergy (2016)

or the reducing end of cellulose, belong to GH families 5, 6, 7, 9, and 48. Finally, β-
glucosidases (EC 3.2.1.21) act on soluble cello-oligosaccharides, including cellobiose,
toward the release of D-glucose, belong to GH families 1, 3, 5, 9, 30, and 116. Recently,
members of GH family 61 that were known in literature for many years as ‘weak
endoglucanases’, have been shown to lack GH activity by exhibiting monoxygenase profile, 19-
21 displaying little or no activity on polymeric cellulose.

All cellulolytic enzymes known to date, except GH61s, are GHs that utilize the same catalytic

mechanism of acid–base catalysis, with inversion or retention of D-glucose anomeric
configuration. The cellulase active sites are categorized in two common types. GHs with open
(groove, cleft) active sites, which typically exhibit endocellulase activity (endo-hydrolases)
that bind almost anywhere along the length of the cellulose polysaccharide and hydrolyze the
β-1,4 glycosidic bonds between D-glucose units, whereas there are also tunnel-like active
sites that exhibit exocellulase activity (exo-hydrolases) that bind at the ends of the cellulose
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polysaccharide producing specific oligosaccharide products (Table 2.1). Exocellulolytic
enzymes are typically processive enzymes, which means that they are attached to the cellulose
chain until it is completely hydrolyzed, whereas endocellulases can be both processive and
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nonprocessive enzymes. The efficiency of the processive cellulases can greatly influence the
rate-limiting step of cellulose hydrolysis. 23

Table 2.1 Structural Conservation and Diversity of Cellulases